Cell cycle progression, cell migrations and gene expression that occur at the wrong time or place, or that are inaccurately regulated, can result in abnormal development and can lead to disease states. There are many systems that can be studied to improve our understanding of the cell cycle, cell movements and gene expression. One that has several unique advantages is the early Drosophila embryo when nuclear cycles produce approximately four thousand nuclei within the first two hours after fertilization. These early cycles offer one of the few tractable systems with both high uniformity and an abbreviated and apparently simplified cell cycle. This proposal describes an investigation of these nuclear divisions; findings that are the basis for the proposed experiments already reveal that our understanding of this early, critical stage of development must change radically.